export default function (file,callback,imgWidth,imgHeight){
    let fileObj = file;
    let fname = file.name;
    let ftype = file.type;
    
    let reader = new FileReader();
    reader.readAsDataURL(fileObj); //转base64
    reader.onload = function(e) {
        let image = new Image(); //新建一个img标签（还没嵌入DOM节点)
        image.src = e.target.result;
        image.onload = function() {
            let canvas = document.createElement("canvas"), // 新建canvas
                context = canvas.getContext("2d"),
                imageWidth = Number(imgWidth) || image.width / 2, //压缩后图片的大小
                imageHeight = Number(imgHeight) || image.height / 2,
                data = "";
            canvas.width = imageWidth;
            canvas.height = imageHeight;
            context.drawImage(image, 0, 0, imageWidth, imageHeight);
            data = canvas.toDataURL("image/jpeg"); // 输出压缩后的base64
            let arr = data.split(","),
                mime = arr[0].match(/:(.*?);/)[1], // 转成blob
                bstr = atob(arr[1]),
                n = bstr.length,
                u8arr = new Uint8Array(n);
            while (n--) {
                u8arr[n] = bstr.charCodeAt(n);
            }
            let files = new Blob(
                [new Blob([u8arr], { type: mime })],
                { type: ftype }
            ); // 转成file
            callback(files); // 回调
        };
    };
};
